MODULATORY EFFECTS OF REGIONAL CORTICAL ACTIVATION ON THE ONSET RESPONSES OF THE CAT MEDIAL GENICULATE NEURONS

Corticofugal modulation on activity of thr medial geniculate body (MGB ) was examined by locally activating the primary auditory cortex (AI) and looking for effects on the onset responses of MGB neurons to acous tic stimuli. Of 103 MGB neurons recorded from 13 hemispheres of 11 ani mals, 91 neurons (88%) showed either a facilitatory or inhibitory effe ct or both. of these neurons, 72 showed facilitatory effects and 25 in hibitory effects. The average facilitatory effect was large, with a me an increase of 62.4%. Small inhibitory effects (mean: -16.2%) were obt ained from a few neurons (6 of 103) when a pure tone stimulus was used . whereas the effect became larger and more frequent when a noise burs t stimulus was used (mean: -27.3%: n = 22 of 27 neurons). Activation o f an AI site having the same best frequency (BF) as the MGB neuron bei ng recorded from produced mainly a facilitatory effect on MGB neuronal responses to pure tones. Activation of AI at a site neighboring the B F site produced inhibitory effects on the MGB response when noise burs t stimuli were used. We found that the effective stimulation sites in AI that could modulate MGB activity formed patchlike maps with a diame ter of 1.13 +/- 0.09 (SE) mm (range 0.6-1.9 mm, n = 15) being larger t han the patches of thalamocortical terminal fields. Examining the effe cts of sound intensities, of is neurons tested 9 neurons showed a larg er effect for low-sound-intensity stimuli and small or no effects for high-sound-intensity stimuli. These were named low-sound-intensity eff ective neurons. Five neurons showed high sound intensity effectiveness and four were non-intensity specific. Most low-sound-intensity effect ive neurons were monotonic rate-intensity function neurons. The AI cor tical modulatory effect was frequency specific, because 15 of 27 neuro ns showed a larger facilitatory effect when a BF stimulus was used rat her than a stimulus of any other frequency. The corticothalamic connec tion between the recording site La MGB and the most effective stimulat ion site in AI was confirmed by injecting wheat germ agglutinin-horser adish peroxidase tracer at the stimulation site and producing a small lesion In the recording site. The results suggest that 1) the large fa cilitation effects obtained by AT activation at the region that direct ly projected to the MGB could be the result mainly of the direct proje ction terminals to the MGB relay neurons; 2) the large size patches of the effective stimulation site in AT could be due to widely ramifying corticothalamic projections; and 3) the corticofugal projection selec tively gates auditory information mainly by a facilitatory effect, alt hough there is also an inhibitory effect that depends on the sound sti mulus used.